FIG. 12 illustrates a typical process in the prior arts for forming a ring-shaped work having a ring-like groove on its inner surface (such as an outer race of a ball bearing). A ring-shaped work (which will be called merely "work" hereinafter) 202 is pressed between a roll 203 and a mandrel 206 at the outer and inner surfaces thereof, rolled due to the rotation of the roll 203 and the mandrel 206, and formed a groove 200 on its inner surface by a forming portion having a convex profile 208 formed on the mandrel 206.
In the method described above, forming a deep groove on the work 202 in one step may become difficult since a too drastic deformation of the work 202 causes a decrease in mechanical accuracy of the work 202. Therefore, as shown in FIG. 11, a deep groove has been formed according to such a procedure comprising two or more steps of subsequent rolling of a relatively small degree of deformation. Such procedure, however, requires a plurality of rolling apparatus corresponding to the number of the forming steps, thereby causing an increase in the cost and space of the production facilities. Furthermore, the productivity may decrease since loading and unloading of the work 202 to the apparatus should be repeated for each step of the rolling.
The object of this invention is to offer an apparatus for rolling a ring-shaped work which improves the efficiency of such multistep rolling process and the mechanical accuracy of the work.